This invention relates to a driver element for inductive loads, in particular of a type comprising bridge connected power transistors having respective protection diodes.
A known device of this type is shown, for example, in FIG. 1. This prior device comprises a bridge of four transistors designated at T1,T2,T3 and T4, and four flyback diodes, designated at D1,D2,D3 and D4, each disposed in parallel with a respective transistor. In detail, each diode is connected with its anode to the emitter of the respective transistor and with its cathode to the collector thereof. The load L is arranged at the central branch of the bridge and is flown by a current whose direction and value can be controlled through the transistors T1-T4. Between the base and the emitter of each transistor T1-T4 there is a resistor, whilst between the collector of each transistor T1-T4 and the substrate there is a substrate diode indicated at DS.
The circuit shown is effective to drive inductive loads of various types, such as DC motors, step motors, solenoids, etc., and in particular the flyback diodes D1-D4 are necessary in the recirculation phase to avoid the second reverse breakdown of the transistors.
The transistors receive their driving signal on their respective bases. It is possible to have different on and off sequences of the transistors T1-T4 with the exception of simultaneous switching on of T1 and T2 or T3 and T4. For the explanation of a typical operation it is assumed that the transistor pair T1 and T4 are off, whilst the transistor pair T2 and T3 are on, and in particular that the transistor T3 is continuously fed, whereas the transistor T2 is switching on and off. Switching on of the pair T2-T3 results in the current I.sub.L flowing through the load in the direction of the arrow in FIG. 1, thereby the current drawn by T3 is passed through the load and then discharged through T2. If T2 is switched off, the load current can no longer flow to ground and is forced to recirculate through the diode D1. At that phase, the anode voltage of D1 is higher than the cathode voltage thereof, thereby the base-emitter junction of the transistor T1 will be reverse biased. If the voltage across the diode D1 exceeds 0.6 V, then the collectorbase junction of transistor T1 is likely to be forward biased and cause the transistor T1 to be switched on in the reverse active region. This behavior is disadvantageous because, on switching the transistor T2 back on, the latter will be forced to draw both the load current I.sub.L and the switching-off current of T1 and D1. In that situation, the behavior would be as brought out by the curve I in FIG. 2, showing the current pattern versus time.